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Rebuilt YSM8 having a runaway...any ideas.?

Ok....im a bit stumped by this one, so could someone please help me out....

i just reinstalled a YSM8 after putting in new rings and rod bearings (and friction discs in tranny the main reason for pulling)
The old top ring was right at spec, but others where still in, and rod bearing had only a hint of copper showing sleeve was in spec as well, so motor wasn't worn out.
fast forward to today, i reinstall the engine with new rings and rod bearing fill with oil and do the pre-start routine, she starts up pretty as you please, being as i was by my self i was starting it down in cabin so i could watch for trouble...good thing i did...first start i only ran it about 30 sec....second time i start it and about a minute in i was lowering the throttle and she startda runaway....fortuitously i was by the engine, so grabbed the first thing at hand to stuff in breather tube...which happened to be by bed sheet....but im a bit stumped as ive heard of runaways on old worn engines but on 1 with good rings? im at a lose...anyone have any ideas.?

we did have the front cover off the engine where the governor parts are located to clean out the gunk, but touched nothing in there that i think might have caused this....
there was oil in the crank case breather tube but this was after the engine had sucked on my sheets a bit so i do not know if that was the symptom or a cause

im just worried i might have messed something up in the front of motor where governor is located i touched nothing and it went together ok, but still worried.